The present invention relates to a jib crane and, more specifically, to a jib crane which prevents a burden from being unexpectedly displaced upon dynamic lift off or laying-down of the burden.
FIG. 1 is a side view showing a jib crane in which reference numeral 1 denotes a traveling or stationary type (shown in the figure is the traveling type) support base; and 2, a revolving frame revolvably mounted on the support base 1 via a revolving table 3. The support base 1 and revolving frame 2 constitute a crane body 4.
The revolving frame 2 in the crane body 4 has a front portion to which a jib 5 is pivoted for its luffing movement through a support pin 6. Mounted on the revolving frame 2 is a luffing winch 7 which reels or unreels a luffing rope 8 which in turn is reeved on a sheave 10a at a top of an A-frame 9 on the revolving frame 2, on a sheave 10b at a tip of the jib 5 and again on the sheave 10a and is fixed to the revolving frame 2. The luffing winch 7 reels or unreels the luffing rope 8 to cause luffing motion of the jib 5.
Also mounted on the revolving frame 2 is a hoisting winch 11 which reels or unreels a lifting rope 12 which in turn is reeved on a sheave 13 at the top of the A-frame 9, between the sheave 13 and a sheave 14 (lifting point) at the tip of the jib 5 and between the sheave 14 and a sheave 16 of a hook block 15. The lifting rope 12 is wound at its end around a luffing drum (not shown), which cooperates with the luffing winch 7, in a direction opposite to that of the latter. Driving the hoisting winch 11 causes a burden 17 suspended from the hook block 15 to be lifted up or down.
The lifting rope 12 is unreeled by the luffing drum when the luffing rope 8 is reeled by the luffing winch 7 to raise the jib 5, and is reeled by the luffing drum when the raised jib 5 is lowered into substantially horizontal, thereby providing level luffing of the burden 17 without changing its height. Number of times of reeving the lifting rope 12 between the sheave at the lifting point 14 and the sheave 13 at the upper end of the A-frame 9 is, for example, doubled against number of times of reeving the lifting rope 12 between the sheave at the lifting point 14 and the sheave 16 of the hook block 15, which prevents load applied by the lifting rope 12 from acting as resistance to the luffing motion of the jib 5 to facilitate the luffing motion of the jib 5 and enable smooth level luffing of the burden 17.
FIG. 1 shows the jib 5 of the jib crane in its most raised position (with a luffing angle xcex8 of the jib 5 to horizontal plane being maximum). In this state, the jib crane can lift up the burden 17 of maximum load (or load rating). When the jib 5 is lowered to substantially horizontal as shown in FIG. 3, load of the burden 17 liftable is decreased in terms of increased moment load.
FIG. 2 shows a typically known A-frame 9 mounted on the revolving frame 2 and comprising a front frame 9a with rigidity and a rear frame 9b with a smaller cross-sectional area and acting as a tension bar. Also in FIG. 1, the front frame 9a is a structure with rigidity and the rear frame 9b is a tension bar with a smaller cross-sectional area.
In the jib crane of FIGS. 1 and 2 and when the jib 5 is substantially horizontal, the front and rear frames 9a and 9b of the A-frame 9 are subjected to compression and tensile loads, respectively. When the jib 5 is raised to lift up the burden 17 of maximum load (or load rating), both the front and rear frames 9a and 9b are subjected to large tensile load T.
The above-mentioned conventional jib crane generally has a following problem. Shown in FIG. 1 by the solid lines is the jib crane with the jib 5 being raised without the burden 17. When the burden 17 of maximum load is lifted up from this state, the crane body 4 and jib 5 are deflected and tilted forward as indicated by chain double-dashed lines due to the heavy load. In other words, the tip of the jib 5 is bent downward and the support base 1 of the crane body 4 is deflected forward and the revolving table 3 is deflected forward.
When the burden of maximum load is lifted up, extremely large tensile load T acts on both the front and rear frames 9a and 9b of the A-frame 9 as shown in FIG. 2; the rear frame 9b of the conventional A-frame 9, which is used as a tension bar and has a smaller cross-sectional area, is lengthened by the tensile load T, resulting in deflection and forward tilting of the entire A-frame 9 as indicated by chain double-dashed lines.
The above-mentioned forward deflections of the crane body 4, jib 5 and A-frame 9 are greatest when the burden 17 of maximum load is lifted up with the jib 5 being raised. When the jib 5 approaches horizontal, a forward displacement distance of the tip of the jib 5 is decreased in connection with reduced load of the burden 17 liftable and the luffing angle xcex8 of the jib 5 from horizontal plane.
As mentioned above, when the burden 17 of maximum load is lifted up by the jib crane, the crane body 4, jib 5 and A-frame 9 are deflected and tilted forward so that the lifting point 14 at the tip of the jib 5 is displaced forward by a forward displacement distance +X as shown in FIG. 1. This causes the burden 17 to be displaced forward, by the forward displacement distance +X, from a position originally expected.
As a result, in the jib crane of FIG. 1 and upon dynamic lift off of the burden 17 of maximum load on the ground with the hook block 15 being aligned to a gravity center of the burden 17, the crane body 4, jib 5 and A-frame 9 are tilted forward as shown by the chain double-dashed lines as mentioned above and the burden 17 is thrown forward by the forward displacement distance +X, resulting in a problem of the burden 17 being swung back and forth.
When the burden 17 of maximum load lifted by the jib crane as indicated by the chain double-dashed lines in FIG. 1 is laid down into a predetermined position, the load of the burden 17 is relieved the very moment the burden 17 contacts the installation position, which causes the forward tilted crane body 4 to be raised up as shown by the solid lines. As a result, the burden 17 is unexpectedly drawn back by the forward displacement distance +X.
Thus, since the burden 17 is displaced when it is dynamically lifted off the ground or laid down, collision of the burden 17 with any nearby structure or other problems may occur. In a case where the burden 17 such as a steel block is lifted up, moved and positioned for placement on an object to be welded, the steel block is displaced the very moment it is placed on the object to be welded, which results in difficulties in accurately positioning the block and causes a problem of a long time being required for the positioning work.
An object of the invention is to provide a jib crane wherein cross-sectional areas of upper and lower beam members constituting a jib are determined depending upon overhang eccentric lengths of the upper and lower beam members such that, when a burden of maximum load is lifted up, an upper portion of the jib is recurved toward a crane body and thus a forward displacement distance of a tip of the jib due to forward tilting of the crane body is counterbalanced with a backward displacement distance due to the recurvature of the jib toward the crane body, thereby preventing unexpected displacement of the burden when the burden is dynamically lifted off a ground or laid down by the jib crane.
A further object of the invention is to provide a jib crane wherein cross-sectional areas of front and rear frames constituting an A-frame are determined such that, when a burden of maximum load is lifted up, the front frame is lengthened to displace backward an upper end of the A-frame and thus a forward displacement distance of a tip of the jib due to forward tilting of a crane body is counterbalanced with a backward displacement distance due to the backward deformation of the A-frame, thereby preventing unexpected displacement of the burden when the burden is dynamically lifted off a ground or laid down by the jib crane.
A still further object of the invention is to provide a jib crane wherein determination of cross-sectional areas of upper and lower beam members constituting a jib depending upon overhang eccentric lengths of the upper and lower beam members is carried out concurrently with determination of cross-sectional areas of front and rear frames constituting an A-frame, thereby preventing a tip of the jib from being displaced when a burden of maximum load is lifted up.